1. Field of the Invention
This invention relates to a urea melting technique. This invention particularly relates to a process for producing zinc sulfide particles with the urea melting technique.
2. Description of the Related Art
Zinc sulfide phosphors have heretofore been utilized for luminous paint compositions, cathode ray tubes, fluorescent plates of electronic microscopes, electroluminescence (EL) devices, and the like. As an activator, silver, copper, manganese, or the like, has heretofore been added to the zinc sulfide phosphors. Also, as a co-activator, aluminum, a halogen, or the like, has heretofore been added to the zinc sulfide phosphors.
As one of techniques for producing zinc sulfide, there has heretofore been known a technique, wherein hydrogen sulfide is introduced to a saturation point into an aqueous zinc oxide solution, which is kept in an acidic state by use of a buffer, and zinc sulfide is thus precipitated. As another technique for producing zinc sulfide, there has heretofore been known a technique, wherein thiourea, or the like, is added to an aqueous alkali solution containing zinc ions and caused to undergo a reaction.
However, with the aforesaid conventional techniques for producing zinc sulfide, zinc sulfide particles having particle diameters distributed over a wide range are obtained. In order for uniform particle size to be obtained, it is necessary for various adjustments, such as a pH adjustment, to be performed during the production processing. Also, in cases where an activator or a co-activator is added to zinc sulfide having been obtained with the aforesaid conventional techniques for producing zinc sulfide, and the resulting mixture is subjected to heat treatment, abnormal particle growth is apt to occur, and a zinc sulfide phosphor having uniform particle size is not always capable of being obtained. Further, the zinc sulfide phosphors are markedly apt to suffer from adverse effects of impurities, and light emission intensity of the zinc sulfide phosphors becomes low in cases where 10−6 mol of iron, cobalt, nickel, or the like, mixes into the zinc sulfide phosphors. However, with the aforesaid conventional techniques for producing zinc sulfide, wherein zinc sulfide crystals are precipitated from the aqueous solution, impurities readily mix in to the zinc sulfide phosphors, and it becomes necessary for particular processing for, for example, removing the impurities, to be performed.
A technique for producing a zinc sulfide phosphor, which has a smaller particle diameter and more uniform particle diameter and exhibits a higher luminance than those of the zinc sulfide phosphors having been produced with the aforesaid conventional techniques for producing zinc sulfide, is described in, for example, Japanese Patent Publication No. 4(1992)-58517. The technique for producing a zinc sulfide phosphor, which is described in Japanese Patent Publication No. 4(1992)-58517, comprises a first step of mixing and melting thiourea, a zinc salt, and an activator in order to prepare a molten salt, a second step of subjecting at least either one of ammonia and an alkali hydroxide to a reaction with the molten salt, zinc sulfide containing the activator being thereby precipitated, and a third step of firing the zinc sulfide precipitate under an inert atmosphere or a sulfurizing atmosphere.
The technique for producing a zinc sulfide phosphor, which is described in Japanese Patent Publication No. 4(1992)-58517, comprises the second step of subjecting at least either one of ammonia and an alkali hydroxide to a reaction with the thiourea melt, zinc sulfide containing the activator being thereby precipitated. With the second step described above, a precipitant, such as an ammonia gas or the alkali hydroxide, is added to the thiourea melt, and zinc sulfide containing the activator is thereby precipitated. However, in the cases of the reaction utilizing the precipitant described above, the problems occur in that, if the precipitant is not uniformly added to the thiourea melt, uniformity of the zinc sulfide phosphor will not be capable of being obtained.